Device in a force actuator

ABSTRACT

A reciprocating axial movement of a drive member (18) in a force actuator is generated by a prestressed tool (17) of Terfenol material being exposed to a pulsating magnetic field. The reciprocating movement is transformed to an axial movement in an application direction of a push rod (21) coaxial therewith. The drive member and the push rod are connected by a connection member (22), which is in self-locking thread engagement with a drive member and the push road and is subjected to a torque from a spiral spring (23) for its rotation during the idle stroke of the reciprocating movement in the direction for advancing the push rod in the application direction.

TECHNICAL FIELD

This invention relates to a device in a force actuator for transforminga reciprocating axial movement of a drive member to an axial movement inan application direction of a push rod coaxial therewith.

TECHNICAL BACKGROUND

In a number of different ways it is possible to accomplish areciprocating movement or force, which it may be desirable to transforminto a undirectional movement or force. A relatively recently developedtechnique is to utilize materials having magnetostrictive properties.Magnetostriction means that a material change its dimensions whenexposed to a magnetic field.

Materials with such properties are typically alloys with rare earthmetals, an alloy with terbium and iron being available under the nameTerfenol. This material Terfenol has a very high magnetostriction,namely up to 0.2% at a magnetic field with a field strength of appr.3000 oersted, and a very fast response.

Due to the magnetostriction a Terfenol rod exposed to a pulsatingmagnetic field and prestressed for accomplishing a return to originalshape will have a reciprocating length and accordingly exert areciprocating force.

In order to utilize this oscillating movement or force in a practicalforce actuator this movement has to be transformed into a unidirectionalmovement of a push rod or the like in the actuator.

THE INVENTION

In a force actuator of the kink described above this may according tothe invention be accomplished in that the drive member and the push rodare connected by means of a connection member, which is in self-lockingthread engagement with the drive member and/or the push rod and issubjected to a torque for its rotation during the idle stroke of thereciprocating movement in the direction for advancing the push rod inthe application direction.

In a first practical embodiment a connection ring is in threadengagement with external threads on the drive member and is subjected toa torque exerted by a prestressed spring. In a preferred embodiment theconnection ring is in thread engagement with oppositely directed threadson the drive member and the push rod.

In a second practical embodiment a connection rod is in threadengagement with oppositely directed internal threads in the drive memberand the push rod and is subjected to a torque excerted by a prestressedspring.

In these embodiments a clock or spiral spring is arranged between theconnection member and an outer ring, which is rotatable, preferably mymeans of an electric motor, for prestressing the spring.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in further detail below reference beingmade to the accompanying drawings, in which

FIG. 1 shows a rather simple embodiment of a disc brake with a actuatorillustrating the principle underlying the invention,

FIG. 2 is a side-view, mostly in section, of a preferred embodiment ofan actuator including a movement transforming device according to theinvention, and

FIGS. 3 and 4 are side-views, partly in section, of two modified devicesaccording to the invention.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1 shows a simple disc brake arrangement, which is more intended toillustrate a principle than a practical embodiment.

The arrangement is intended to brake a rotating disc 1 by means of brakepads 2 arranged at the ends of two caliper levers 3 and 4 pivotallyattached to each other at 5. The braking action is obtained by pushingthe ends of the levers 3 and 4 opposite the pads 2 apart.

In the simple arrangement according to FIG. 1 this is accomplished by anelectric coil 6 surrounding a rod 7 of the material called Terfenol,with the characteristics to prolong itself up to say 0.2% of its lengthwhen subjected to a certain magnetic field (from the coil 6). The rod 7is connected to the lever 3 to the left in FIG. 1 and cooperates withthe lever 4 to the right via a wedge 8. The wedge 8 is biassed upwardsby a compression spring 9 and cooperates with a wedge surface in thelever 9.

The coil 6 is electrically activated in a pulsating manner. The massinertia of the levers 3 and 4 is considerably larger than that of therod 7 and the self-locking mechanism constituted by the wedge 8 andspring 9. Each time the coil 6 is activated, the rod 7 is slightlyextended with the result that the levers 3 and 4 are pressed apart. Whenthe other the rod 7 returns to its original length due to thede-activation of the coil 6, the wedge 8 will be free for a short whileand will be pushed upwards in FIG. 1 by its spring 9. Depending on thefrequency of the pulsating currency supplied to the coil 6 and thegeometry of the arrangement, the brake pads 2 will be applied to thedisc 1 with a certain force and speed.

For releasing the brake the coil may again be activated by a pulsatingcurrent, but the wedge 8 will in this instance be pushed back (againstthe force of the compression spring 9) by for example a solenoid (notshown).

Having now illustrated the principle of the actuator based on theTerfenol material, time has come to describe a practical and preferredembodiment, shown in FIG. 2.

This actuator has a housing 10 with a lid 11 attached thereto. The lid11 is provided with a fixed ear 12, whereas a movable ear 13 extends outof the housing 10 opposite the fixed ear 12. These ears are to beconnected to a disc brake arrangement of the same general type asillustrated in FIG. 1.

A cylindrical permanent magnet 14 is arranged within the housing 10supported by a partition-wall 15 therein, and a cylindrical coil 16 isprovided within the permanent magnet. A rod 17 of Terfenol is arrangedwithin the coil 16.

Coaxially in line with the rod 17 is a drive member in the form of adrive sleeve 18. A pull bar 19 provided with end flanges at both endsextends through holes in the drive sleeve 18, the rod 17 and the lid 11.A compression spring 20 (in the form of a number of belleville springs)is arranged between the lid 11 and the end flange of the bar 19 for thepurpose of excerting a pull force in the bar 19 and accordinglyproviding a prestress in the Terfenol rod 17, which is essential for itsintended function.

A movement transforming device for this actuator has the followingdesign;

Coaxially in line with the drive sleeve 18 is a push rod 21, whichextends out through the housing 10 and is ended by the ear 13. When theear 13 is connected to a brake rigging (not shown), the push rod 21 isnon-rotatable. The drive sleeve 18 and the push rod 21 are provided withexternal threads, that are oppositely directed and cooperate withcorresponding internal threads in a connection member in the form of aconnection ring 22. The respective threads are self-locking but have apitch that is as great as possible in order to minimize the powerrequired. The connection ring 22 should have a minimal mass inertia formaking it easily movable.

The connection ring 22 is normally exposed to a torque (in the directionfor pushing the sleeve 18 and rod 21 apart) by means of a clock springor spiral spring 23. This spring 23 is at its inner end attached to theconnection ring 22 and at its outer end to an outer ring 24. This outerring 24 may be rotated in either direction by means of an electric motor25 connected by a pinion 26 to a gear ring 27 at the outer peripherypurpose to keep the spring 23 tensioned at all times and the secondarypurpose to rewind the connection ring 22 by rotation in the directionopposite to that for tensioning the spring. When not operating theelectric motor 25 is locked.

The basis for the operation of the disclosed actuator is that theprestressed rod 17 of Terfenol is caused to reciprocate by means of apulsating magnetic field. Further, there are self-locking means in theform of the connection ring 22 only requiring a small outer force forits operation. The reciprocation frequency for the rod 17 has to begreater than the natural frequency of the brake rigging to which theactuator is connected.

The following discussion is based on the assumptions that an actuator ofthis kind with a Terfenol rod 17 as the force-generating means has tohave an application stroke of some 3-4 mm, an application time of 0.1 sand an application force of 10,000N.

The specific property of the Terfenol material is that its lengthincreases when exposed to a magnetic field; the property is calledmagnetostriction. The material chosen has a high magnetostriction,namely up to 0.2%, and a very fast response. The material has to beprestressed in order to return to its original dimension when notexposed to the magnetic field.

Calculations have shown that under the following conditions the desiredresults may be obtained:

A rod 17 with a length of 100 mm and a diameter of 20 mm is under amechanical prestress of 3700N by the spring 20. This rod is caused toreciprocate at a frequency of 1000 by being exposed to the combinationof a constant magnetic field from the permanent magnet 14 of 130 kA/mand pulsating magnetic field from the coil 16 of ±130 kA/m; theresulting positive magnetic field pulsates between zero and 260 kA/m.

For obtaining this magnetic field the coil 16 may have an inner diameterof 22 mm, an outer diameter of 40 mm and a length of 110 mm. The currentmay be 5 A and the voltage 625 V for obtaining the desired magneticfield with a climbing time of 0.25 ms and a power of 806 W.

Provided that the mass of the brake rigging levers is 15 kg, the force10,000N and the time for reaching this force 0.25 ms, the final speedwill be 83 mm/s and the movement 0.007 mm.

It has been demonstrated that the Terfenol rod 17 reciprocates under theinfluence of the magnetic field and delivers the desired forces. Thisreciprocating movement of the drive sleeve 18 is to be transferred to aundirectional application movement of the push rod 21 by means of theself-locking connection ring 22. The simple principle thereof is to benon-rotational during each small reciprocation movement in theapplication direction of the Terfenol rod 17 and to increase thedistance between the drive sleeve 18 and the push rod 21 by smallrotation during each small reciprocation movement in the retractiondirection. This small rotation of the connection ring 22 is accomplishedby the spring 23.

It is essential that the connection ring 22 has a very low mass inertialand a high thread pitch (in opposite directions). With an outer diameterof 30 mm and an inner diameter of 20 mm and a pitch of 7 mm/turn atorque of less than 4 Nm from the spring 23 is required.

As has already been described, the spring 23 is tensioned by theelectric motor 25 or by similar means. The return stroke (by rotatingthe connection ring 22 in the opposite direction) can also be obtainedby the electric motor 25. A more direction transmission of thisrotational movement than by the spring 23 can be obtained by providing aone-way clutch between the connection ring 22 and the outer ring 24.

Two modified versions of the movement transforming device are shown inFIGS. 3 and 4.

In the FIG. 3 modification there is a drive member or sleeve 30, a pushrod 31, a connection ring 32, a clock or spiral spring 33 and an outerring 34. The oscillating drive sleeve 30 is in similar thread engagementwith the connection ring 32 as in the FIG. 2 version, whereas theconnection ring is freely rotatable in relation to, but axiallyconnected to the push rod 31, which for this purpose is provided with ahead 31' engaging a corresponding recess in the connection ring 32.

In the FIG. 4 modification the drive member 40 and the push rod 41 areprovided with internal threads for self-locking cooperation withexternal threads of a connection member in the form of a connection rod42. A clock or spiral spring 43 joins this rod 42 with an outer ring 44.

We claim:
 1. A device in a force actuator for transforming areciprocating axial movement of a drive member to an axial movement inan application direction of a push rod member of a braking systemcoaxial therewith, comprising in combination means for transferringforces between the drive member and the push rod member by means ofconnection member rotatable in self-locking thread engagement with oneof the members to convert an advancing and idle axial stroke of thedrive member into incremental axial movements of the push rod, rotatingmeans subjecting the connection member to a rotational torque rotatingduring the idle stroke of the reciprocating movement in a direction foradvancing the push rod in the brake application direction, reciprocatingmeans for a drive member comprising a prestressed rod of amagnetostrictive material, a surrounding coil, and means for producing apulsating magnetic field in said coil.
 2. A device according to claim 1,wherein said connection member further comprises a connection ring inthread engagement with external thread on the drive member and saidrotating means further comprises a prestressed spiral spring.
 3. Adevice according to claim 2, wherein the connection ring is in threadengagement with oppositely directed threads on the drive member and thepush rod member.
 4. A device according to claim 1, wherein saidconnection member further comprises a connection rod in threadengagement with oppositely directed threads of the drive member and thepush rod member and said rotating means further comprise a prestressedspring.
 5. A device according to claim 1 wherein said rotating meansfurther comprises a spiral spring with positioning means comprising arotatable outer ring coupled for prestressing the spring.
 6. A deviceaccording to claim 5, further comprising means for rotating the outerring by means of an electric motor.
 7. A device according to claim 1,further comprising reciprocating means for the drive member comprising aprestressed rod of a magnetostrictive material a surrounding coil, andmeans for producing a pulsating magnetic field in said coil.
 8. A brakesystem with a brake applying push rod comprising in combination aprestressed magnetostrictive rod, a surrounding coil, means forproducing a pulsating magnetic field in said coil to reciprocate thelength of the magnetostrictive rod and a mechanism for converting thereciprocations in length of the magnetostrictive rod to incrementalbrake applying movements of said push rod.